I think I've been around just about every language on the block, though not always for production stuff. I totally agree with the comment that one can learn much from other languages even though their capabilities overlap. Paradigms are often clearer in one language over another. For instance, objects and references are much clearer in Ruby than they are in Perl, but CPAN is much dearer (so far) than RubyGems, especially for sysadmin type tasks. Ruby's OO is much like the Smalltalk I used to love, only better, and I find myself buliding objects naturally in Ruby where it's work in Perl.

C is essential to understand UNIX. It's also necessary to understand modules built with XS and swig. Look especially at make. It's not a 'real' programming language, but it's a really useful tool.

Another useful tool for sysadmins, though you can equal its functionality with Net:SSH et al, is expect. expect is just the cat's meow for using Perl to massage other systems as though you were sitting at the keyboard. Highly recommended, and, of course, there's a Perl module to talk to it!